The present invention relates to a copper foil for printed circuits, more specifically, the present invention relates to a copper foil for printed circuits, in which powder fall from the copper foil can be reduced and the peeling strength and heat resistance can be improved, prepared by forming a primary particle layer of copper on a surface of a copper foil, and then forming a secondary particle layer based on copper-cobalt-nickel alloy plating thereon. The copper foil for printed circuits of the present invention is particularly suitable for, for example, a fine-pattern printed circuit and a FPC (Flexible Printed Circuit) for magnetic heads.
Copper and copper alloy foils (hereinafter referred to as “copper foil”) have contributed significantly to the advances in the electric- and electronic-related industries and become essential, in particular, as materials for printed circuits. In general, a copper clad laminate is prepared by adhesively laminating a copper foil for printed circuits onto a substrate such as a synthetic resin board or film under high temperature and high pressure with or without using an adhesive. Then a necessary circuit is printed via the steps of resist application and exposure, followed by etching treatment for eliminating unwanted portions to form an intended circuit. Finally, desired elements are soldered to form various types of printed circuit boards for electronics devices. Although a treating method of a copper foil for printed circuit boards is different between a surface (roughened surface) to be bonded to the resin base material, and a non-bonding surface (glossy surface), a number of methods have been proposed for respective surfaces.
For example, main requirements for the roughened surface formed on a copper foil include: (1) no oxidative discoloration during storage; (2) sufficient peel strength against a substrate even after high-temperature heating, wet processing, soldering, chemical treatment and the like; and (3) no so-called lamination stains, which may occur after the lamination onto a substrate and etching treatment. Roughening treatment of copper foil has an important role in determining the adhesiveness between the copper foil and the substrate. For this roughening treatment, copper roughening treatment in which copper is electrodeposited was initially employed, but then various other technologies have been proposed. Now, copper-nickel roughening treatment has become established as one of the representative treatment methods in order to improve thermal peeling strength, hydrochloric-acid resistance and oxidation resistance. The present applicant has proposed copper-nickel roughening treatment (see Japanese Laid-Open Patent Publication No. S52-145769 A), and produced successful results. A copper-nickel treated surface takes on a black color, and this black color of the copper-nickel treated surface is now acknowledged as a product symbol of, in particular, rolled foil for flexible substrates.
The copper-nickel roughening treatment is superior in terms of thermal peeling strength, oxidation resistance and hydrochloric acid resistance; but etching with an alkali etching solution, which has become increasingly important for fine pattern treatment in recent years, becomes difficult, etch residues remain on the treated layer during the formation of fine patterns having 150 μm pitch or less. Accordingly, for fine pattern treatment, the present applicant previously developed Cu—Co treatment (see Japanese Patent Publication No. S63-2158 and Japanese Patent Application No. H01-112227 A) and Cu—Co—Ni treatment (see Japanese Patent Application No. H01-112226 A). These roughening treatments showed better etching properties, alkali etching properties and hydrochloric-acid resistance, but it was found that thermal peeling strength was decreased when an acrylic adhesive was used, and oxidation resistance was not sufficient as desired, and the color tone did not become black but was brown to dark brown.
Pursuant to the trend of finer patterns and diversification of printed circuits in recent years, the followings are further required: 1) thermal peeling strength (particularly when an acrylic adhesive is used) and hydrochloric-acid resistance should be comparable to those for the case of the Cu—Ni treatment; 2) etching with an alkali etching solution should be possible to produce a printed circuit having a pattern of 150 μm pitch or less; 3) oxidation resistance should be improved (being resistant to oxidation in an oven at 180° C. for 30 minutes) as in the case of the Cu—Ni treatment; and 4) the treatment should produce a black surface as in the case of the Cu—Ni treatment.
In other words, when the circuit becomes finer, the circuit tends to more easily peel off due to a hydrochloric acid etching solution, so that such peeling off needs to be prevented. When the circuit becomes finer, the circuit also tends to more easily peel off due to the high temperature at a process such as soldering, so that such peeling off also needs to be prevented. At the present time when circuits are increasingly becoming finer, etching a printed circuit having a pattern of 150 μm pitch or less with the use of, for example, a CuCl2 etching solution is already an essential requirement, and alkali etching is also becoming a necessary requirement because resists and the like are increasingly varied. A black surface is also becoming more important from the viewpoint of manufacturing copper foil and chip mounting in order to improve positioning precision and heat absorption.
In response to such demands, the present applicant successfully developed a method of treating a copper foil, wherein a cobalt plated layer or a cobalt-nickel alloy plated layer is formed on the copper foil surface after being subject to copper roughening treatment via a copper-cobalt-nickel alloy plating, so that: the copper foil possesses many of the above general properties as the copper foil for printed circuits and particularly possesses the properties described above which are comparable to those in the Cu—Ni treatment; thermal peeling strength is not reduced when using an acrylic adhesive; the copper foil has superior oxidation resistance; and the copper foil surface takes on a blackened color (see Japanese Patent Publication No. H06-54831). Preferably, after forming the cobalt plated layer or the cobalt-nickel alloy plated layer, rustproof treatment is applied, represented by coating treatment with chrome oxide alone or coating treatment with the mixture of chrome oxide and zinc and/or zinc oxide.
Subsequently, as the development of electronic equipment advances, semiconductor devices increasingly become smaller and more highly integrated. Even higher temperature is required for treatments in the manufacturing process of these printed circuits, and heat is generated when using a product in which such semiconductor devices are incorporated. As a result, the decrease in bonding strength between a copper foil and a resin substrate is again recognized as a problem. In light of the above, the present applicant inventively improved thermal peeling resistance in the method for treating copper foil for printed circuits according to Japanese Patent Publication No. 2849059, in which a roughening treatment is performed to a surface of a copper foil by way of copper-cobalt-nickel alloy plating, and a cobalt plated layer or a cobalt-nickel alloy plated layer is thereafter formed.
This method for treating copper foil for printed circuits is a method in which a cobalt-nickel alloy plated layer is formed after performing a roughening treatment to a surface of a copper foil via copper-cobalt-nickel alloy plating, and then a zinc-nickel alloy plated layer is further formed. It is an extremely effective invention, and materials produced according to the invention are now one of the major products of copper-foil circuit materials. For producing a copper-foil circuit that is even more thin-lined, the process of soft etching the upper surface of a copper circuit using an etching solution containing sulfuric acid and hydrogen peroxide is performed after a circuit is once formed on a substrate. During this process, a problem occurred that the etching solution infiltrated into the edge portion of the bonded interface between the copper foil and a resin substrate such as polyimide. This may be considered as a partial corrosion of the treated surface of the copper foil. Such corrosion is a significant problem since it reduces the bonding strength between the copper foil and the resin in a fine circuit. Solving this problem is also demanded.
In the treatment of copper foil for printed circuits, in which a cobalt-nickel alloy plated layer is formed after performing a roughening treatment to a surface of a copper foil via copper-cobalt-nickel alloy plating, and then a zinc-nickel alloy plated layer is further formed; the present inventors, after numerous attempts, successfully provided several major improvements in the properties of the copper foil for printed circuits. The early technologies of the roughening treatment by copper-cobalt-nickel alloy plating are disclosed in Japanese Laid-Open Patent Publication No. H04-96395 and Japanese Laid-Open Patent Publication No. H10-018075 A.
However, since the structure of the roughening particles formed on the surface of copper foil via copper-cobalt-nickel alloy plating as the most basic roughening treatment is dendritic, there was a problem that the particles came off from the upper or base parts of the dendritic structure, causing a phenomenon generally known as powder fall. This powder fall is a problematic phenomenon. Although a roughened layer treated by copper-cobalt-nickel alloy plating can be characterized by having superior adherence with a resin layer and superior heat resistance, the particles therein may easily come off by external force as mentioned above, causing flaking due to “rubbing”, adhesion of flakes onto the roll, and occurrence of etch residues due to the flakes during the process.
Provided that blackening treatment by copper-cobalt-nickel alloy plating (in Example 1, Cu: 3.3 mg/dm2, Co: 6.3 mg/dm2, Ni: 1.6 mg/dm2) is performed, techniques are disclosed in which copper foil is plated with fine copper particles in advance in order to make the color tone darker in the blackening treatment and to prevent the phenomenon of powder fall, and in which a smooth layer of cobalt or cobalt-nickel is provided as the outermost layer to prevent powder fall (Japanese Laid-Open Patent Publication No. 2004-260068 A). In this case, providing a smooth layer of cobalt or cobalt-nickel as the outermost layer is the main requirement for preventing powder fall. Rather, powder fall from copper-cobalt-nickel alloy plating will depend on the structure of particles in a primary layer of copper applied to copper foil, and the composition and the particle structure of a cobalt layer or a cobalt-nickel layer formed as a secondary particle layer thereon. However, Japanese Laid-Open Patent Publication No. 2004-260068 only provides formation of a smooth layer as the outermost layer, hardly providing a fundamental solution for powder fall.